Mechanism of Colloidal Attachment on Textile Fibrous Media Mehanizem

achieved an mechanism. Filtration Abstract Filtration through the porous media of a granular bed is one of the oldest and most favourable particle separation techniques used universally for the treatment of water. In the case of ﬁ ltration through a granular bed, all physical factors are incorporated into a single collector contact eﬃ ciency. The latter is the ratio of the rate at which particles strike the collector to the rate at which particles ﬂ ow towards the collector. On the other hand, collision or attachment eﬃ ciency represents the chemical interaction between the media used and colloids, and is expressed as the ratio of the number of particles removed by the collector to the number of particle collector collisions, or the possibility that a collision in an attachment. Textile media is emerging as a substrate in deep bed ﬁ ltration due to its superior performance in the removal of colloidal particles from water under higher ﬁ ltration velocities compared with granular media. Reported studies relating to the eﬀ ect of physicochemical factors on colloidal removal in textile ﬁ brous media are mainly based on the experimental value of the concentration variation of colloidal particles in input and output water. Presented in this paper is a way in which colloidal ﬁ ltration theory can be extended to textile ﬁ lter media in order to explain the mechanism of attachment of colloids (primarily bacteria) on a textile ﬁ brous media surface.

Tekstilec, 2019, 62 (2), [101][102][103][104][105][106][107][108][109] Mechanism of Colloidal Attachment on Textile Fibrous Media is an eff ective process for the removal of colloids from surface water using an attachment mechanism [3]. Signifi cant eff orts have been made by a number of researchers in the development of colloidal fi ltration theory (mainly sand, glass bed, etc., as mentioned in theory) based on the physicochemical interaction between colloids and media [4−6]. Th e main limitations in granular media are the capacity to retain colloidal particles within the pore spaces and a low fi ltration rate [7]. However, granular fi ltration theory is well developed based on the physicochemical interaction of the bed and colloidal particles. Textile materials have emerged in recent years as a substrate that can be used as fi lter media for the removal of colloidal particles from surface water using an attachment mechanism [8]. Superior performance in the removal of colloidal particles from water can be achieved by textile fi brous media under fi ltration velocities that are ten times higher than granular media [9]. Th ere are several varieties of textile fi bre available on the market that can be used as fi lter media. However, the effi cient removal of colloidal particles from surface water using a textile fi lter relies heavily on the type of fi bres used as fi ltering media. Most textile fi ltration research is based on the eff ect of the shape, size, thickness, hydrophobicity and surface charge of textile media on fi ltration effi ciency [10]. Th ese factors of textile fi brous media fi lters are similar to the physicochemical factors of granular media that serve as the basis of an attachment mechanism. A quantitative analysis of textile fi brous media may be possible by extending the physicochemical interaction concept of granular media. Th e physicochemical factors of textile materials, such as shape, size, surface charge, the hydrophobicity of fi bres, and media thickness and colloidal properties, determine the strength of colloidal-fi brous media physicochemical interactions [11]. Presented in this paper is a potential way to extend colloidal fi ltration theory to textile fi brous media in order to explain the mechanism of attachment of colloids (bacteria) on a textile media surface.

Filtration theory
Bacteria attachment in porous media as predicted by colloid fi ltration theory provides a model for determining the physical and chemical factors of particle retention in porous media. Physical controls over bacteria attachment in porous media depend on the geometry of the porous media, as well as bacteria.
Th e number of potential contacts of particles with a surface can be estimated based on a physical mechanism. Th is, in turn, facilitates the determination of the attachment of particles in porous media, which depends on the chemical forces aff ecting adhesion and repulsion [12]. Particle removal in a packed bed in a constant state using an attachment mechanism can be described using one-dimensional fi ltration equation 1 [13].
where C is particle concentration (in number of bacterial per unit volume), L is the thickness of the fi lter bed, (1-f) is the solid fraction, η is the single collector contact effi ciency and d c is the collector diameter. Integration of the thickness of the packed bed yield is given in basic fi ltration equation 2.
Where F p is fractional penetration and this is an indicator of the balance between cell adsorption and desorption. Physical factors that account for particle collisions with porous media are incorporated into the single collector contact effi ciency (η). Th e single collector contact effi ciency of a single media particle or collector (η) is a ratio, i.e. the rate at which particles strike the collector to the rate at which particles fl ow towards the collector. Numerous analytical solutions have been used to specify the single collector contact effi ciency for fi ltration through granular media. Primarily transport mechanisms are used to develop the model for the theoretical calculation of single collector contact effi ciency. Th e Yao model, represented by equations 3−5 describing deep bed fi ltration for liquid fi ltration, were proposed by Logan et al. [14]. Single collector contact effi ciencies for this model are based on spherical collectors.
Here, η D , η I and η G represent theoretical values for the single collector contact effi ciency when the sole Single collector contact effi ciencies are a dimensionless number and are developed from correlations using equations 7−9.
Where P e is the peclet number, R and G are interception and gravitational numbers, U 0 and U p are fi lter super facial velocity (L/T) and particle settling velocity (L/T), D is particle diff usivity (L 2 /T), d p is the particles diameter and d c is the collector diameter. Th e particle settling velocity is obtained using equation 10.
μ and ϑ, dynamic and kinematic viscosity (M L -1 T -1 ) of fl uid g is the gravitational constant (L/T), where p p and p f the particle and fl uid density (M/L 3 ). Th e particle diff usivity is obtained using the Stokes-Einstein equation (equation 11).
Th e quantitative assessment of bacterial attachment to a collector surface is carried out by determining the collision effi ciency (attachment) factor (α), and is oft en expressed as the ratio of experimental single collector effi ciency (η), calculated using equation 2, to the predicted single collector effi ciency (η o ), calculated using equation 6, or the possibility that a collision in an attachment, which is obtained using equation 12.
2 Physicochemical factors of textile fi brous media aff ecting the removal of colloidal particles

Fibre size/diameter
In textile porous media, the attachment of colloids on the surface of a fi bre is infl uenced by the fi bre diameter. It has been reported that a lower fi bre diameter will result in the higher removal effi ciency of colloids due to the high specifi c surface area and good interconnectivity of pores [15]. Th e mean pore size of a fi brous media is highly dependent on the fi bre diameter [16]. Eichhorn and Sampson used a theoretical model to demonstrate that the fi bre diameter plays an important role in controlling the pore size of an electrospun nanofi brous network used for water fi ltration [17]. Zhou et al. [18] studied the removal of colloidal particles in cellulose acetate nanofi bers membranes at diff erent fi bre diameters. Th ey found that membranes with a lower fi bre diameter had a higher removal effi ciency than membranes with a higher fi bre diameter. Desai et al. [19] studied the bacteria removal effi ciency of nanofi bers fi lter media by varying the diameter of the fi bre. Th ey reported that an increase in fi bre diameter resulted in a decrease in fi ltration effi ciency. It has been reported that the fi bre diameter in textile fi lter media plays an important role in improving fi ltration effi ciency.

Fibre shape/cross section
Many studies have been carried out on the eff ect of the shape of the fi bre in textile media on colloidal removal in the fi ltration process. A higher projected surface area resulting from a diff erent cross-section facilitates the probability of capturing colloids particles. Recently, hollow nanofi ber membranes have emerged as substrates for use in liquid fi ltration [20]. Wang et al. [21] found that hollow fi bre membranes have excellent intrinsic separation properties due to their highly porous and narrow pore size distribution, which leads to high fi ltration effi ciency. Fibre cross-section could also be considered an important factor in colloidal fi ltration in textile fi lter media.

Fibre media thickness
Th e thickness of the media also aff ects the removal effi ciency of a fi lter. Kaur et al. [22] suggested that if a fi brous media is used to separate sub-microns particles, a thicker fi brous layer is required to reduce the overall average pore size of the media. A higher media thickness is associated with increased colloidal removal due to the overlapping of the fibres in the media, resulting in fi ne sized pores, which facilitates the trapping of particles [18]. It has also been reported that the removal of clay particles increases with an increase in the thickness of the polypropylene fi brous barrier. It was claimed that a thicker barrier should provide more chances for the interception of clay particles [23−25].

Fibre hydrophobicity
Th e hydrophobicity of textile fi bres is considered an important chemical factor that aff ects colloidal removal in textile fi brous media. Most colloidal particles are hydrophobic in nature. Hydrophobic interaction plays an important role in the effi cient removal of colloidal particles [26]. Th e kinetics of the capture of colloidal particles is also determined by the magnitude of the hydrophobic interaction between particles and collectors. Hydrophobic interaction increases with an increase in the hydrophobicity of the fi lter media. Th e hydrophobicity of the media is characterised in terms of contact angle value [11]. Arnold et al. [27] reported that hydrophobicity is directly proportional to the contact angle of water with its surface and inversely proportional to the work of adhesion. Fletcher et al. [28] found a strong positive correlation between the number of bacteria attached to the surface and the hydrophobic nature of polymers, as determined by the contact angle. It is evident in Figure 1 that an increase in the contact angle results in increased bacteria attachment. Pringle et al. [29] calculated a lower work of adhesion for nylon (98 mJ/m 2 ) than for glass (146 mJ/m 2 ). In an experiment with two pseudomonas species, it was observed that the attachment of these cells was higher to the nylon fi bres than to the borosilicate glass surface because of the higher hydrophobicity of the nylon fi bre.

Fibre surface charge
Th e removal of colloidal particles using relatively wide pore size fi brous media can be infl uenced not only by sieving parameters (pore size and pore size distribution) but also by the chemical interaction taking place between the colloidal particles and fi brous media [30,31]. Cookson et al. [32] reported that attachment is brought about by the colloid-media chemical interaction controlled by the surface properties of the respective materials. Surface charge is one of the most important surface properties controlling the eff ective attachment of colloids on the fi bre surface in textile fibrous media. It was reported that a possible mechanism for the removal of smaller-sized colloidal particles could be the electrostatic attraction between the opposite charges of the fi bres and the particles, which causes the deposition of particles on the fi bre surface [33−35]. Th e fi bre surface charge is characterised in terms of zeta potential. It is measured using a streaming potential method on the surface of the fi bres. Th e surface charges of fi bres are the result of the disassociation of fi bre surface groups in an aqueous medium. Th e isoelectric point (IEP) is the pH value corresponding to zero zeta potential and is diff erent from fi bre to fi bre, depending on the surface properties [36]. Kang et al. [33] studied the adsorption of negatively charged nanoparticles on cationic, surfactanttreated microporous polypropylene fi lters. Th ey reported that fi ltration effi ciency can be increased from 10% to 90% through this surface modifi cation. Th is can be attributed to the lowering energy barrier between the particles and fi lter media. Druet et al. [37] investigated the removal effi ciency of heavy metal using positively charged, chitosan-treated polyethylene terephthalate geotextiles. Th ey claimed that the higher positive charge of the media at an acidic pH will result in higher metal removal effi ciency. In the desalination process based on textile nanofi bers membranes, most membranes are characterised by a negative surface Figure 1: Relationship between bacteria attachment and the water contact angle on materials [28] Tekstilec, 2019, 62 (2), 101-109

Figure 4: Eff ect of fi bre orientation on single collector contact effi ciency and approach velocity [42]
To quantitatively compare removal effi ciency with diff erent fi bre orientations under identical solution conditions, the value of collision (attachment) efficiency (α) is calculated using equation no. 12, which is used in colloidal fi ltration theory. It has been reported that a high fi bre orientation angle may lead to the exposure of a greater surface area for the striking of bacteria, resulting in high collision effi ciency. A higher collision effi ciency means higher bacteria attachment, where its maximum value is 1. It is evident from Figure 5 that an increase in fi bre orientation results in an increase in collision effi ciency.

Eff ect of fi bre mass on bacteria removal effi ciency
In another study conducted by Roy et al [43] on the eff ect of fi bre mass on bacterial attachment, it was determined that removal effi ciency increases with an increase in media mass up to a certain level (Figure 6). Th is is due to the change in single collector Mechanism of Colloidal Attachment on Textile Fibrous Media charge to enhance the removal of dissolved salts [38,39]. Berg et al. [40] found that the electrostatic repulsion force of negatively charged pesticides on the surface of a negatively charged membrane is expected to enhance the overall removal effi ciency. Based on the above literature, it can be concluded that the physicochemical factors of textile fi brous media are identifi ed on the basis of fi ltration effi ciency, which is expressed in terms of the concentration variation of colloidal particles in input and output water. Th e effect of physicochemical factors on the mechanism of attachment of colloidal particles, primarily bacteria on textile fi brous media, may also be systematically investigated by using colloidal fi ltration theory.
3 Selected approaches to the application of colloidal fi ltration theory for textile material An attempt has been made by a few researchers to use colloidal fi ltration theory to explain colloidal removal by an attachment mechanism in textile porous media. Dagaonkar et al. [41] used DLVO theory to explore the eff ect of solution chemistry on colloidal removal in nonwoven polyester fi lter fabric. Th ey reported that under unfavourable attachment conditions using bivalent salt (CaCl 2 ), removal efficiency increased from 35% to 62% compared to monovalent salt (NaCl), while the removal effi ciency remain constant at around 38% for the ionic strength range of 0 to 100 mM ( Figure 2).

Eff ect of fi bre orientation on bacteria fi ltration
Roy et al [42] investigated the attachment of bacteria to fibrous material as a function of fibre orientation to the direction of the liquid flow. Removal trends were explained on the basis of colloidal filtration theory. They reported that by changing fibre orientation from 0° to 90°, bacteria removal efficiency increased from 30% to 54.54%, suggesting that the attachment of bacteria on the media surface depends on fibre orientation ( Figure 3) Figure 3: Removal effi ciency as a function of the fi bre orientation angle [42] It is evident from Figure 4 that single collector efficiency increases from 1.33×10 -2 to 1.39×10 -2 by changing fi bre orientation from 0° to 90°. Th is is due to a decrement in the approach velocity of the fi ltration system from 3.09×10 -3 to 2.01×10 -3 m/s. Enhanced single collector contact effi ciency by increasing fi bre orientation has therefore been attributed to a change in the approach velocity of water in the fi ltration system. A potential explanation is that a high fi bre orientation angle may lead to the exposure of a greater surface area for the striking of bacteria, resulting in high collector contact effi ciency. contact effi ciency and attachment/collision effi ciency, as observed from experimental data regarding removal effi ciency. It is evident from Figure 7 that changing the media mass increases and then decreases collision efficiency (α).

Eff ect of diff erent fi brous material on bacterial attachment
Many researches have studied the eff ect of diff erent media material on bacterial attachment in textile fi brous media. Roy et al. [44] reported that nylon fi brous media demonstrates a higher removal efficiency than polyester fi brous media for the same solution chemistry (Figure 8). Th e bacteria removal effi ciency of nylon and polyester fi brous fi lter media are explained based on colloidal fi ltration theory. Th e removal and attachment of bacteria on a fi brous surface thus increases with an increase in ionic strength, which can be attributed to a change in collision effi ciency. It is evident from Figure 9 that the change in collision effi ciency is higher for nylon than for polyester, resulting in a higher removal efficiency by nylon fi brous media.

Conclusion
Th e reported study demonstrated that the fi bre orientation of the fi lter media may play an important role in bacterial attachment. Bacteria attachment and removal effi ciency increase with an increase in the fi bre orientation angle. According to colloidal fi ltration theory, this is possible due to a change in the collision (attachment) effi ciency of the fi brous media. Bacteria attachment and removal effi ciency increase with an increase in media mass up to a certain level. It is also evident from reported studies that media materials play an important role in bacterial attachment in a fibrous packed bed. Th e higher bacteria removal effi ciency of nylon fibrous media than polyester fi brous media is due to the higher collision effi ciency of nylon fi bre (0.24 to 0.46) than polyester fi bre (0.23 to 0.42) at the ionic strength range of 1mM to 150 mM. Hence, the concept of dimensionless α (attachment effi ciency) or η (single collector contact effi ciency) of a granular media fi lter can be extended to textile fi lter media for the purpose of explaining the mechanism of attachment of colloids (bacteria) on textile fi brous media.   [44]